Only a few examples of antisense transcripts have been extensively characterized in the human genome and they are often associated with imprinted loci. In an Advanced Online Publication in Nature Biotechnology Rodrigo Yelin and colleagues at Compugen Ltd in Israel propose that regulation by antisense transcription may be quite frequent (Nature Biotechnology, 17 March 2003, doi:10.1038/nbt808). They used a computational approach (the 'Antisensor' algorithm) to identify 2,667 human genomic loci with evidence of transcripts from both strands. Microarray analysis with strand-specific oligonucleotide probes demonstrated that as many as 60% of these may be true sense-antisense pairs. This places the number of possible antisense-regulated genes much higher than previous estimates.